Players will be able to sit in those, but zombies can grab them out of them and the player would have to repeatably click a button to avoid being pulled off. Spare parts can be used to make them mode secure. 
Regarding zombie interaction with vehicles; that's too complicated. It would be extremely difficult, if not impossible, to detect whether or not a player is exposed in a vehicle while taking into consideration just how "improved" that vehicle has been made. The more practical way for zombie interaction would be to allow zombies to do damage to a vehicle and when damage exceeds a certain amount, zombies can enter the vehicle.
Any sort of "building", vehicles or structures, will be a feature to be added later in DayB's lifetime. As of now, vehicles will only function much like I described to MyNameIsYou's question earlier.